Inefficient delivery and poor uptake of therapeutic drugs to solid tumors hamper the efficacy of cancer treatments. Therefore, the “enhanced permeability and retention” (EPR) effect of solid tumors has been explored extensively as a target in the design of drug delivery systems. Solid tumors behave differently from normal tissues, having several abnormalities, such as leaky blood vessels and a poor lymph system. It is an important feature that nanosized particles can extravasate from the vasculature and passively accumulate in tumors. Inorganic nanoparticles, especially gold nanoparticles (Au NPs) with good biocompatibility, versatile surfaces, tunable sizes, and unique optical properties have received significant attention as drug delivery systems to improve targeting effect and efficacy for cancer treatments.